Conventionally, a semiconductor device is available which includes a semiconductor stacked structure including an electron transit layer and an electron supply layer provided over a substrate.
In such a semiconductor device as just described, since an n-type impurity is doped in the electron supply layer that functions also as a barrier layer, energy of a conduction band sometimes drops at the portion at which the n-type impurity is doped and a channel by which electrons are conducted is formed also in the electron supply layer. In particular, a situation in which electrons are conducted in the electron supply layer other than the electron transit layer, namely, so-called parallel conduction, sometimes occurs.
Since a channel in the electron supply layer by which electrons are conducted has an electron mobility and an electronic speed lower than those in the electron transit layer, if a channel by which electrons are conducted is formed in the electron supply layer, namely, if the parallel conduction occurs, then degradation in characteristic is caused.
Thus, a technology is available that, by setting a composition of Al in a region in which an n-type impurity is to be doped higher than a composition of Al in a non-doped region, for example, in an AlGaN barrier layer, energy of the conduction band of the region in which the n-type impurity is to be doped is increased and decrease of the energy of the conduction band arising from doping with the n-type impurity is compensated for. This is referred to as first technology.
It is to be noted that also a configuration is available that an InAlAs layer having a high composition of Al is provided on the opposite sides across an InAlAs electron supply layer in which the n-type impurity is doped entirely.